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C� m W clean at the 10 % level (in which 1 in 10 canisters is certified after cleaning)or at the 100 % level (in which every canister is certified after cleaning). The cleaning process is the same for both certification levels and utilizes dilution, heat, and high vacuum. The certification process utilizes EPA Method TO-15 (GC/MS) to ensure that VOC concentrations are <1 ppb,,. Only canisters certified at the 100 % level can be used for soil vapor sampling activities for human health risk assessment. The use of 100 % certified canisters is required in order to minimize potential interferences in analyzing low concentrations for human health risk assessment. Both 1 L and 6 L canisters can be certified at the 100 % level. For sub-slab soil vapor sampling, use only 1 L canisters in order to minimize the volume of soil gas collected. Large sub-slab sample volume increases the possibility of capturing vapor from soils some distance below the slab, rather than vapor in granular materials directly underlying the slab. For all other soil vapor samples, the use of 6 L canisters is recommended due to variability between laboratories in required vapor aliquots for analysis. Some labs may be able to obtain two sample aliquots from a 1 L canister(in case sample dilution and a second analytical run is required), but many labs require 500 mL or 1 L sample aliquots, in which case 6 L canisters must be used. Before ordering 1 L canisters, please contact the laboratory to inquire about required sample aliquots and achievable reporting limits from 1 L canisters. After cleaning, the canister is evacuated until a pressure of-29.9 in Hg is obtained. The canister will hold a vacuum of<-25 in Hg for more than 30 days. The maximum holding time for canisters after sampling is completed is variable: for California, the maximum holding time is 72 hours (California EPA, 2003), while the EPA specifies 14 days (USEPA, 1999). Check the locally applicable regulations to determine the maximum holding time for the site in question, but do not exceed 14 days. As discussed below, the soil vapor sample flows into the canister due to the pressure gradient between the vadose zone and the canister. A flow controller/particulate filter, provided by the laboratory, controls the vapor flow rate into the canister. Figure 10. 6 L Summa canister(image from Air Toxics Ltd.) ETC Soil Vapor Sampling Technical Toolkit 16 Version 1.4, September 7,2005